The principle works the same for the chords from a minor scale, although the symbols are slightly different (because the chords have different qualities).

For example, the key of A minor:

Am

Bdim

C

Dm

E7

F

G7

i

ii°

III

iv

V7

VI

VII7

It is also possible to use Roman numerals to describe chords that are not diatonic. In other words, chords that are borrowed from other keys.

For example, the chord bIII is the 3rd chord (III), in major (uppercase letters), lowered by a half-step (b). In the key of C, this would be the Eb major chord.

You may also see a sharp symbol combined with a Roman numeral: #IV in the key of C is the F# major chord.

It is not uncommon to add a qualifier to the Roman numeral. Examples: IVmaj7, II7, #IVdim7. To find the real chord, substitute the Roman numeral for the n-th chord from the scale.

You may have heard of the Nashville Number System. This is the same principle, although it works with plain-old numbers instead of Roman numerals. So instead of II-V-I you’d see 2-5-1, but they both mean the same thing.

Solfege is yet another system, except that it doesn’t use numbers, but syllables:

1

2

3

4

5

6

7

Do

Re

Mi

Fa

Sol

La

Ti

And finally, each of the diatonic chords can also be given a name that more-or-less describes its function. Different chords have different functions in their key. I’ll keep the details for a future article, so I’ll simply give you the list here:

1

Tonic

2

Supertonic

3

Mediant

4

Subdominant

5

Dominant

6

Submediant

7

Leading tone (or subtonic)

So now you know that when people talk about the “I-chord” or “tonic”, they mean the first chord from the key.